It may seem a little embarrassing: you wake up and your pillow is drenched in slobber. After you wipe the spit at the corners of your mouth, you might wonder: Why do we drool in our sleep?

The body normally produces more than 1 liter of saliva per day. It is produced by salivary glands and is usually swallowed and re-circulated via the bloodstream. Drooling occurs when the saliva collects within the mouth and rather than being swallowed, it drips or runs out past the lips. Why might this occur more at night?

The muscles of the body relax during sleep, especially during REM sleep, and it is possible for your mouth to fall open at this time. One of the biggest reasons your mouth could come open at night is because you can’t breathe well through your nose. We are meant to breathe through our nose, but if congestion is present, our default option is to breathe through the mouth. If this occurs in sleep, saliva may drool onto our pillow.